1. Field of the Invention
This invention relates to a sheet through reading type image reading apparatus to read documents passing through a contact glass surface and more particularly to an image reading apparatus to convey documents stably on a contact glass surface.
2. Description of the Related Art
A sheet through type image reading apparatus that is used in an image forming apparatus for image reading reads documents which are conveyed by an automatic document feeder and sequentially passing on a contact glass surface using a stationary image sensor. When documents are read with this image sensor, read images are blurred by the changes in the passing position of documents on a contact glass surface and the quality of read images are varied. In particular, in the case of color image documents, the quality of read image is largely affected by the change in passing position of documents on a contact glass surface and the image quality may be deteriorated remarkably depending on the passing position of documents on a contact glass surface.
In view of the above, a technology to set the reading position of an image reading apparatus corresponding to changes in the passing position of documents on a contact glass surface is so far proposed as disclosed in Japanese Patent Application No. 2002-247300.
However, in the conventional technology described above, it is necessary to compute a passing position of documents on a contract glass surface from sampling data using documents for use in adjustment when the image reading portion is moved and set. Therefore, the moving and setting operation of the image reading portion was complicated.
Accordingly, in the sheet through reading type image reading apparatus, it is desirable to make the operation easy and retain read images in stabilized quality irrespective of conditions for conveying documents to a contact glass surface.